According to the basic system and process for recovering VOC's and solvents from circulating process air, activated carbon beds or other adsorption material filter beds are utilized in alternating adsorption and desorption cycles. The volatile organic chemicals and solvents are removed from exhaust air circulating from a chemical process plant or manufacturing process area by passing the process air through the adsorption filter bed. The VOC's and solvents are adsorbed onto the surfaces of the adsorption bed during the adsorb or purification cycle. The purified or purged process air can then be recirculated through the chemical or manufacturing plant or be otherwise exhausted.
When the adsorption bed such as an activated carbon bed is saturated with adsorbed solvents and VOC's, the inlet and outlet ducts of the bed are switched to other lines for the desorption or regeneration cycle. A desorbing inert gas such as nitrogen is heated and circulated through the adsorption bed to desorb solvents and VOC's into the desorption inert gas flow. The solvents and VOC's are then recovered from the desorb inert gas flow in a condenser through which cooling liquid is circulated. Solvent and VOC condensate is drained from the condenser to a storage tank. The inert gas such as nitrogen is recirculated from the condenser for the continuing desorption and regeneration of the adsorption filter bed. Typically, two adsorption beds are provided, alternating in opposite phases between adsorb and desorb cycles so that one adsorption bed is always available for removing VOC's from exhaust air.
A number of United States Patents were found describing variations and enhancements on this basic system and method. The Sacchetti et al. U.S. Pat. No. 4,421,532 describes a process for removing and recovering volatile organic substances from industrial waste gases. The Sacchetti et al. patent describes the addition of an unusual step in the desorption or regeneration cycle. After circulating heated nitrogen gas through the activated carbon bed, the N.sub.2 flow is cooled followed by condensing and recovering the solvents or VOC's. According to the step of Sacchetti et al. however the cooled nitrogen gas is then passed through the adsorbent bed to remove residual solvents and VOC's. The nitrogen gas is then reheated and the desorption cycle continues. Sacchetti et al. thus recover solvents and VOC's from the inert desorption gas partly by condensation in the condensing/cooling step and partly by readsorption onto the surfaces of the activated carbon bed.
The Winter U.S. Pat. No. 4,516,988 describes a method and apparatus for purifying a gas stream in a sorption filter in which the volumetric flow of gas to be purified is combined with a volumetric flow of circulating purified gas which is larger than the volumetric flow of the gas to be purified. Winter contends that this blending of volumetric flows is more effective in achieving and maintaining a desired or specified emission value. The Winter et al. U.S., Pat. No. 4,516,988 is directed to the problem of selective adsorption of vaporous or gaseous impurities, that is to the separation of the solvents and VOC's from each other. Winter et al. accomplishes this by providing two filter beds in tandem or in series and maintaining the two filter beds at two different temperatures. The first filter bed is maintained at a sufficiently high temperature to adsorb solvents or VOC's of low volatility while passing through the impurities of higher volatility. The second adsorption filter is maintained at a lower temperature than the first adsorption filter to adsorb the impurities of higher volatility. Winter et al. are therefore able to achieve fractional separation.
The Blaudszun U.S., Pat. No. 4,414,003 describes another process and apparatus for the recovery of solvents incorporating a positive compression step of compressing the inert desorbing gas after it is "laden" with solvent. The compressed inert gas stream laden with solvent vapors is cooled "in an expansion turbine while delivering work". Blaudszun uses a two bed system for maintaining oxygen concentration in the stream of nitrogen gas "below the maximally admissible level". According to the Blaudszun process, the flow of nitrogen gas through the saturated activated carbon bed during the desorption cycle may be diverted through the second activated carbon bed in series in order to maintain the sensed oxygen concentration "below the maximally admissible level".
A disadvantage of such existing VOC recovery systems is that hazardous conditions may occur during either adsorption or desorption cycles. For example upon initiation of a desorption phase, a threshold combination of high temperatures, VOC concentrations, and oxygen (O.sub.2) may create combustible conditions resulting in fire or explosive reactions. Similarly, during an adsorption phase, excessively high VOC concentration and temperatures may also create combustible conditions. Fire and other hazardous conditions may spread between the VOC recovery system and the plant source of process air.
Another disadvantage of conventional VOC recovery systems is that such systems may operate inefficiently. System parameters may not be maintained at preferred values for optimum performance and efficiency. Furthermore, such systems are not flexibly adaptable to different environments and applications for recovering different VOC's and solvents.